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<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast"><tt>const_cast</tt></a> - <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Memory allocation</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new"> <code>new</code> expression</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete"> <code>delete</code> expression</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Classes</a></td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class.html" title="cpp/language/class"> Class declaration</a></td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> Constructors</strong> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this"> <code>this</code> pointer</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access"> Access specifiers</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"> <code>friend</code> specifier</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Class-specific function properties</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual"> Virtual function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"> <code>override</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"> <code>final</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><tt>explicit</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static"><tt>static</tt></a> </td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor"> Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor"> Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor"> Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect"> Copy assignment</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect"> Move assignment</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor"> Destructor</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"><a href="templates.html" title="cpp/language/templates"> Templates</a> </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> Class template </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> Function template </a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> Template specialization</a></td></tr>   
<tr class="t-nv"><td colspan="5"> <a href="parameter_pack.html" title="cpp/language/parameter pack"> Parameter packs</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Miscellaneous </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> Inline assembly</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="history.html" title="cpp/language/history"> History of C++</a></td></tr>
</table></div></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="classes.html" title="cpp/language/classes"> Classes</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> General </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Overview</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class.html" title="cpp/language/class"> <code>class</code>/<code>struct</code> types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="union.html" title="cpp/language/union"> <code>union</code> types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="injected-class-name.html" title="cpp/language/injected-class-name"> Injected-class-name</a> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Members </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="data_members.html" title="cpp/language/data members">Data members</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static">Static members</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this">The <code>this</code> pointer</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="nested_classes.html" title="cpp/language/nested classes" class="mw-redirect">Nested classes</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="member_template.html" title="cpp/language/member template">Member templates</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="bitfield.html" title="cpp/language/bit field">Bit fields</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="using_declaration.html" title="cpp/language/using declaration"><code>using</code>-declarations</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="member_functions.html" title="cpp/language/member functions">Member functions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access">Member access specifiers</a></td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink">Constructors and member initializer lists</strong></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="data_members.html#Member_initialization" title="cpp/language/data members">Default member initializer</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"><code>friend</code> specifier</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><code>explicit</code> specifier</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="converting_constructor.html" title="cpp/language/converting constructor">Converting constructor</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor">Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor">Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor">Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/copy assignment">Copy assignment operator</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move assignment">Move assignment operator</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor">Destructor</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Inheritance </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="derived_class.html" title="cpp/language/derived class">Base and derived classes</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="ebo.html" title="cpp/language/ebo">Empty base optimization (EBO)</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual">Virtual member functions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="abstract_class.html" title="cpp/language/abstract class">Pure virtual functions and abstract classes</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"><tt>override</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"><tt>final</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Constructor is a special non-static <a href="member_functions.html" title="cpp/language/member functions">member function</a> of a class that is used to initialize objects of its class type.
</p><p>In the definition of a constructor of a class, <i>member initializer list</i> specifies the initializers for direct and virtual bases and non-static data members.
(Not to be confused with <span class="t-lc"><a href="../utility/initializer_list.html" title="cpp/utility/initializer list">std::initializer_list</a></span>.)
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<p>A constructor must not be a <a href="coroutines.html" title="cpp/language/coroutines">coroutine</a>.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx20">(since C++20)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Syntax">Syntax</span></h3>
<p>Constructors are declared using member <a href="function.html" title="cpp/language/function">function declarators</a> of the following form:
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">class-name</span> <code><b>(</b></code> <span class="t-spar">parameter-list</span><span class="t-mark">(optional)</span> <code><b>)</b></code> <span class="t-spar">except-spec</span><span class="t-mark">(optional)</span> <span class="t-spar">attr</span><span class="t-mark">(optional)</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>Where <span class="t-spar">class-name</span> must name the current class (or current instantiation of a class template), or, when declared at namespace scope or in a friend declaration, it must be a qualified class name.
</p><p>The only specifiers allowed in the <span class="t-spar">decl-specifier-seq</span> of a constructor declaration are <a href="friend.html" title="cpp/language/friend"><code>friend</code></a>, <a href="inline.html" title="cpp/language/inline"><code>inline</code></a><span class="t-rev-inl t-since-cxx11"><span>, <a href="constexpr.html" title="cpp/language/constexpr"><code>constexpr</code></a></span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span><span class="t-rev-inl t-since-cxx20"><span>, <a href="consteval.html" title="cpp/language/consteval"><code>consteval</code></a></span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span>, and <a href="explicit.html" title="cpp/language/explicit"><code>explicit</code></a> (in particular, no return type is allowed). Note that <a href="member_functions.html" title="cpp/language/member functions">cv- and ref-qualifiers</a> are not allowed either: const and volatile semantics of an object under construction don't kick in until the most-derived constructor completes.
</p><p>The body of a <a href="function.html" title="cpp/language/function">function definition</a> of any constructor, before the opening brace of the compound statement, may include the <i>member initializer list</i>, whose syntax is the colon character <code><b>:</b></code>, followed by the comma-separated list of one or more <span class="t-spar">member-initializers</span>, each of which has the following syntax:
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">class-or-identifier</span> <code><b>(</b></code> <span class="t-spar">expression-list</span><span class="t-mark">(optional)</span> <code><b>)</b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">class-or-identifier</span> <span class="t-spar">braced-init-list</span>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">parameter-pack</span> <code><b>...</b></code>
</td>
<td> (3)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> Initializes the base or member named by <span class="t-spar">class-or-identifier</span> using <a href="direct_initialization.html" title="cpp/language/direct initialization">direct-initialization</a> or, if <span class="t-spar">expression-list</span> is empty, <a href="value_initialization.html" title="cpp/language/value initialization">value-initialization</a></div>
<div class="t-li1"><span class="t-li">2)</span> Initializes the base or member named by <span class="t-spar">class-or-identifier</span> using <a href="list_initialization.html" title="cpp/language/list initialization">list-initialization</a> (which becomes <a href="value_initialization.html" title="cpp/language/value initialization">value-initialization</a> if the list is empty and <a href="aggregate_initialization.html" title="cpp/language/aggregate initialization">aggregate-initialization</a> when initializing an aggregate)</div>
<div class="t-li1"><span class="t-li">3)</span> Initializes multiple bases using a <a href="parameter_pack.html#Base_specifiers_and_member_initializer_lists" title="cpp/language/parameter pack">pack expansion</a></div>
<table class="t-par-begin">


<tr class="t-par">
<td> <span class="t-spar">class-or-identifier</span>
</td>
<td> -
</td>
<td> any identifier that names a non-static data member or any type name which names either the class itself (for delegating constructors) or a direct or virtual base.
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">expression-list</span>
</td>
<td> -
</td>
<td> possibly empty, comma-separated list of the arguments to pass to the constructor of the base or member
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">braced-init-list</span>
</td>
<td> -
</td>
<td> brace-enclosed list of comma-separated initializers and nested braced-init-lists
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">parameter-pack</span>
</td>
<td> -
</td>
<td> name of a variadic template <a href="parameter_pack.html#Base_specifiers_and_member_initializer_lists" title="cpp/language/parameter pack">parameter pack</a>
</td></tr></table>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S
<span class="br0">{</span>
    <span class="kw4">int</span> n<span class="sy4">;</span>
 
    S<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>       <span class="co1">// constructor declaration</span>
 
    S<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> n<span class="br0">(</span><span class="nu0">7</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// constructor definition:</span>
                  <span class="co1">// ": n(7)" is the initializer list</span>
                  <span class="co1">// ": n(7) {}" is the function body</span>
<span class="br0">}</span><span class="sy4">;</span>
 
S<span class="sy4">::</span><span class="me2">S</span><span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="sy4">:</span> n<span class="br0">{</span>x<span class="br0">}</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// constructor definition: ": n{x}" is the initializer list</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    S s<span class="sy4">;</span>      <span class="co1">// calls S::S()</span>
    S s2<span class="br0">(</span><span class="nu0">10</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls S::S(int)</span>
<span class="br0">}</span></pre></div></div>
</div>
<h3><span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>Constructors have no names and cannot be called directly. They are invoked when <a href="initialization.html" title="cpp/language/initialization">initialization</a> takes place, and they are selected according to the rules of initialization. The constructors without <code>explicit</code> specifier are <a href="converting_constructor.html" title="cpp/language/converting constructor">converting constructors</a>. The constructors with a <code>constexpr</code> specifier make their type a <a href="../named_req/LiteralType.html" title="cpp/named req/LiteralType"><span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">LiteralType</span></a>. Constructors that may be called without any argument are <a href="default_constructor.html" title="cpp/language/default constructor">default constructors</a>. Constructors that take another object of the same type as the argument are <a href="copy_constructor.html" title="cpp/language/copy constructor">copy constructors</a> and <a href="move_constructor.html" title="cpp/language/move constructor">move constructors</a>.
</p><p>Before the compound statement that forms the function body of the constructor begins executing, initialization of all direct bases, virtual bases, and non-static data members is finished. Member initializer list is the place where non-default initialization of these objects can be specified. For bases and non-static data members that cannot be default-initialized, such as members of reference and const-qualified types, member initializers must be specified. No initialization is performed for <a href="union.html#Anonymous_unions" title="cpp/language/union">anonymous unions</a> or <a href="union.html#Union-like_class" title="cpp/language/union">variant members</a> that do not have a member initializer.
</p><p>The initializers where <span class="t-spar">class-or-identifier</span> names a <a href="derived_class.html" title="cpp/language/derived class">virtual base class</a> are ignored during construction of any class that is not the most derived class of the object that's being constructed.
</p><p>Names that appear in <span class="t-spar">expression-list</span> or <span class="t-spar">brace-init-list</span> are evaluated in scope of the constructor:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> X
<span class="br0">{</span>
    <span class="kw4">int</span> a, b, i, j<span class="sy4">;</span>
<span class="kw1">public</span><span class="sy4">:</span>
    <span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&amp;</span> r<span class="sy4">;</span>
    X<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span>
      <span class="sy4">:</span> r<span class="br0">(</span>a<span class="br0">)</span> <span class="co1">// initializes X::r to refer to X::a</span>
      , b<span class="br0">{</span>i<span class="br0">}</span> <span class="co1">// initializes X::b to the value of the parameter i</span>
      , i<span class="br0">(</span>i<span class="br0">)</span> <span class="co1">// initializes X::i to the value of the parameter i</span>
      , j<span class="br0">(</span>this<span class="sy2">-</span><span class="sy1">&gt;</span>i<span class="br0">)</span> <span class="co1">// initializes X::j to the value of X::i</span>
    <span class="br0">{</span><span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>Exceptions that are thrown from member initializers may be handled by <a href="function-try-block.html" title="cpp/language/function-try-block">function-try-block</a>
</p><p>Member functions (including virtual member functions) can be called from member initializers, but the behavior is undefined if not all direct bases are initialized at that point.
</p><p>For virtual calls (if the direct bases are initialized at that point), the same rules apply as the rules for the virtual calls from constructors and destructors: virtual member functions behave as if the dynamic type of <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> is the static type of the class that's being constructed (dynamic dispatch does not propagate down the inheritance hierarchy) and virtual calls (but not static calls) to <a href="abstract_class.html" title="cpp/language/abstract class">pure virtual</a> member functions are undefined behavior.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>If a non-static data member has a <a href="data_members.html#Member_initialization" title="cpp/language/data members">default member initializer</a> and also appears in a member initializer list, then the member initializer is used and the default member initializer is ignored:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S
<span class="br0">{</span>
    <span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span>   <span class="co1">// default member initializer</span>
    S<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> n<span class="br0">(</span><span class="nu0">7</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// will set n to 7, not 42</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<p>Reference members cannot be bound to temporaries in a member initializer list:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A
<span class="br0">{</span>
    A<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> v<span class="br0">(</span><span class="nu0">42</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// Error</span>
    <span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&amp;</span> v<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>Note: same applies to <a href="data_members.html#Member_initialization" title="cpp/language/data members">default member initializer</a>.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<h4> <span class="mw-headline" id="Delegating_constructor">Delegating constructor</span></h4>
<p>If the name of the class itself appears as <span class="t-spar">class-or-identifier</span> in the member initializer list, then the list must consist of that one member initializer only; such a constructor is known as the <i>delegating constructor</i>, and the constructor selected by the only member of the initializer list is the <i>target constructor</i>
</p><p>In this case, the target constructor is selected by overload resolution and executed first, then the control returns to the delegating constructor and its body is executed.
</p><p>Delegating constructors cannot be recursive.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> Foo
<span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span> 
    Foo<span class="br0">(</span><span class="kw4">char</span> x, <span class="kw4">int</span> y<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
    Foo<span class="br0">(</span><span class="kw4">int</span> y<span class="br0">)</span> <span class="sy4">:</span> Foo<span class="br0">(</span><span class="st0">'a'</span>, y<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// Foo(int) delegates to Foo(char, int)</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<h4> <span class="mw-headline" id="Inheriting_constructors">Inheriting constructors</span></h4>
<p>See <a href="using_declaration.html" title="cpp/language/using declaration">using declaration</a>.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<h4><span class="mw-headline" id="Initialization_order">Initialization order</span></h4>
<p>The order of member initializers in the list is irrelevant: the actual order of initialization is as follows:
</p>
<div class="t-li1"><span class="t-li">1)</span> If the constructor is for the most-derived class, virtual bases are initialized in the order in which they appear in depth-first left-to-right traversal of the base class declarations (left-to-right refers to the appearance in base-specifier lists)</div>
<div class="t-li1"><span class="t-li">2)</span> Then, direct bases are initialized in left-to-right order as they appear in this class's base-specifier list</div>
<div class="t-li1"><span class="t-li">3)</span> Then, non-static data member are initialized in order of declaration in the class definition.</div>
<div class="t-li1"><span class="t-li">4)</span> Finally, the body of the constructor is executed</div>
<p>(Note: if initialization order was controlled by the appearance in the member initializer lists of different constructors, then the <a href="destructor.html" title="cpp/language/destructor">destructor</a> wouldn't be able to ensure that the order of destruction is the reverse of the order of construction)
</p>
<h3><span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;fstream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;mutex&gt;</span>
 
<span class="kw1">struct</span> Base
<span class="br0">{</span>
    <span class="kw4">int</span> n<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>   
 
<span class="kw1">struct</span> Class <span class="sy4">:</span> <span class="kw1">public</span> Base
<span class="br0">{</span>
    <span class="kw4">unsigned</span> <span class="kw4">char</span> x<span class="sy4">;</span>
    <span class="kw4">unsigned</span> <span class="kw4">char</span> y<span class="sy4">;</span>
    <a href="../thread/mutex.html"><span class="kw2158">std::<span class="me2">mutex</span></span></a> m<span class="sy4">;</span>
    <a href="../thread/lock_guard.html"><span class="kw2170">std::<span class="me2">lock_guard</span></span></a><span class="sy1">&lt;</span><a href="../thread/mutex.html"><span class="kw2158">std::<span class="me2">mutex</span></span></a><span class="sy1">&gt;</span> lg<span class="sy4">;</span>
    <a href="../io/basic_fstream.html"><span class="kw1738">std::<span class="me2">fstream</span></span></a> f<span class="sy4">;</span>
    <a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a> s<span class="sy4">;</span>
 
    Class<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="sy4">:</span> Base<span class="br0">{</span><span class="nu0">123</span><span class="br0">}</span>, <span class="co1">// initialize base class</span>
        x<span class="br0">(</span>x<span class="br0">)</span>,     <span class="co1">// x (member) is initialized with x (parameter)</span>
        y<span class="br0">{</span><span class="nu0">0</span><span class="br0">}</span>,     <span class="co1">// y initialized to 0</span>
        f<span class="br0">{</span><span class="st0">"test.cc"</span>, std<span class="sy4">::</span><span class="me2">ios</span><span class="sy4">::</span><span class="me2">app</span><span class="br0">}</span>, <span class="co1">// this takes place after m and lg are initialized</span>
        s<span class="br0">(</span>__func__<span class="br0">)</span>, <span class="co1">// __func__ is available because init-list is a part of constructor</span>
        lg<span class="br0">(</span>m<span class="br0">)</span>,    <span class="co1">// lg uses m, which is already initialized</span>
        m<span class="br0">{</span><span class="br0">}</span>       <span class="co1">// m is initialized before lg even though it appears last here</span>
    <span class="br0">{</span><span class="br0">}</span>            <span class="co1">// empty compound statement</span>
 
    Class<span class="br0">(</span><span class="kw4">double</span> a<span class="br0">)</span> <span class="sy4">:</span> y<span class="br0">(</span>a <span class="sy2">+</span> <span class="nu0">1</span><span class="br0">)</span>,
        x<span class="br0">(</span>y<span class="br0">)</span>, <span class="co1">// x will be initialized before y, its value here is indeterminate</span>
        lg<span class="br0">(</span>m<span class="br0">)</span>
    <span class="br0">{</span><span class="br0">}</span> <span class="co1">// base class initializer does not appear in the list, it is</span>
       <span class="co1">// default-initialized (not the same as if Base() were used, which is value-init)</span>
 
    Class<span class="br0">(</span><span class="br0">)</span>
    <span class="kw1">try</span> <span class="co1">// function-try block begins before the function body, which includes init list</span>
      <span class="sy4">:</span> Class<span class="br0">(</span><span class="nu16">0.0</span><span class="br0">)</span> <span class="co1">// delegate constructor</span>
    <span class="br0">{</span>
        <span class="co1">// ...</span>
    <span class="br0">}</span>
    <span class="kw1">catch</span> <span class="br0">(</span>...<span class="br0">)</span>
    <span class="br0">{</span>
        <span class="co1">// exception occurred on initialization</span>
    <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    Class c<span class="sy4">;</span>
    Class c1<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
    Class c2<span class="br0">(</span><span class="nu16">0.1</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
</div>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/194.html">CWG 194</a>
</td>
<td> C++98
</td>
<td> the declarator syntax of constructor only allowed<br>at most one function specifier (e.g. a constructor<br>cannot be declared <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span> <span class="kw1">explicit</span></span></span>)
</td>
<td> multiple function<br>specifiers allowed
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/257.html">CWG 257</a>
</td>
<td> C++98
</td>
<td> it was unspecified whether an abstract class should<br>provide member initializers for its virtual base classes
</td>
<td> specified as not required<br>and such member initializers<br>are ignored during execution
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/263.html">CWG 263</a>
</td>
<td> C++98
</td>
<td> the declarator syntax of constructor<br>prohibited constructors from being friends
</td>
<td> allowed constructors<br>to be friends
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1345.html">CWG 1345</a>
</td>
<td> C++98
</td>
<td> anonymous union members without default<br>member initializers were default-initialized
</td>
<td> they are not initialized
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1435.html">CWG 1435</a>
</td>
<td> C++98
</td>
<td> the meaning of 'class name' in the<br>declarator syntax of constructor was unclear
</td>
<td> changed the syntax to a specialized<br>function declarator syntax
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1696.html">CWG 1696</a>
</td>
<td> C++98
</td>
<td> reference members could be initialized to temporaries<br>(whose lifetime would end at the end of constructor)
</td>
<td> such initialization<br>is ill-formed
</td></tr></table>
<h3><span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-c++20">
<ul><li> C++20 standard (ISO/IEC 14882:2020): 
</li></ul>
<dl><dd><ul><li> 11.4.4 Constructors [class.ctor] 
</li></ul>
</dd></dl>
<dl><dd><ul><li> 11.10.2 Initializing bases and members [class.base.init] 
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++17">
<ul><li> C++17 standard (ISO/IEC 14882:2017): 
</li></ul>
<dl><dd><ul><li> 15.1 Constructors [class.ctor] 
</li></ul>
</dd></dl>
<dl><dd><ul><li> 15.6.2 Initializing bases and members [class.base.init] 
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++14">
<ul><li> C++14 standard (ISO/IEC 14882:2014): 
</li></ul>
<dl><dd><ul><li> 12.1 Constructors [class.ctor] 
</li></ul>
</dd></dl>
<dl><dd><ul><li> 12.6.2 Initializing bases and members [class.base.init] 
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++11">
<ul><li> C++11 standard (ISO/IEC 14882:2011): 
</li></ul>
<dl><dd><ul><li> 12.1 Constructors [class.ctor] 
</li></ul>
</dd></dl>
<dl><dd><ul><li> 12.6.2 Initializing bases and members [class.base.init] 
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++98">
<ul><li> C++98 standard (ISO/IEC 14882:1998): 
</li></ul>
<dl><dd><ul><li> 12.1 Constructors [class.ctor] 
</li></ul>
</dd></dl>
<dl><dd><ul><li> 12.6.2 Initializing bases and members [class.base.init] 
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id="See_also">See also</span></h3>
<ul><li> <a href="copy_elision.html" title="cpp/language/copy elision">copy elision</a>
</li><li> <a href="converting_constructor.html" title="cpp/language/converting constructor">converting constructor</a>
</li><li> <a href="as_operator.html" title="cpp/language/copy assignment">copy assignment</a>
</li><li> <a href="copy_constructor.html" title="cpp/language/copy constructor">copy constructor</a>
</li><li> <a href="default_constructor.html" title="cpp/language/default constructor">default constructor</a>
</li><li> <a href="destructor.html" title="cpp/language/destructor">destructor</a>
</li><li> <a href="explicit.html" title="cpp/language/explicit"><code>explicit</code></a>
</li><li> <a href="initialization.html" title="cpp/language/initialization">initialization</a>
<ul><li> <a href="aggregate_initialization.html" title="cpp/language/aggregate initialization">aggregate initialization</a>
</li><li> <a href="constant_initialization.html" title="cpp/language/constant initialization">constant initialization</a>
</li><li> <a href="copy_initialization.html" title="cpp/language/copy initialization">copy initialization</a>
</li><li> <a href="default_initialization.html" title="cpp/language/default initialization">default initialization</a>
</li><li> <a href="direct_initialization.html" title="cpp/language/direct initialization">direct initialization</a>
</li><li> <a href="list_initialization.html" title="cpp/language/list initialization">list initialization</a>
</li><li> <a href="reference_initialization.html" title="cpp/language/reference initialization">reference initialization</a>
</li><li> <a href="value_initialization.html" title="cpp/language/value initialization">value initialization</a>
</li><li> <a href="zero_initialization.html" title="cpp/language/zero initialization">zero initialization</a>
</li></ul>
</li><li> <a href="move_operator.html" title="cpp/language/move assignment">move assignment</a>
</li><li> <a href="move_constructor.html" title="cpp/language/move constructor">move constructor</a>
</li><li> <a href="new.html" title="cpp/language/new"><code>new</code></a>
</li></ul>

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